This invention relates to a system and method for radiative spraying of polymeric compositions. In particular, the invention relates to a system and method for radiant thermal spray application of polymer foams and other coatings.
A variety of technologies are used to apply polymeric foams and coatings to substrates in the aerospace, aircraft, marine, automotive, appliance, transportation and construction industries. Applications for foams include thermal and acoustical insulation, insulation tiles and components, ablative components, abradable seals, cryogenic applications, water repellant barriers, radomes and firewalls.
In the field of foam application, one approach involves fabricating a preformed foam component by cutting the component from a large bung and attaching the component to the substrate. Another approach involves spraying the foam directly on to the substrate. Coatings are typically sprayed directly on to the substrate. Spraying methods include methods that involve melting the polymer (thermal spray methods) and methods that involve dissolving the polymer in a solvent (solvent spray methods).
Background art methods for applying spray-in-place foams utilize multiple component chemical and solvent based reactions to form the foam. These methods do not incorporate thermal spray technology (e.g., use of high temperatures to process constituent materials into a desired form while spraying onto a substrate).
Related art methods and systems for applying polymer foams and coatings have serious limitations. Manual methods which involve cutting foam components from large bungs are very expensive. Solvent spray methods release toxic (volatile organic compounds, VOC's) to the environment. Related art thermal spray methods which relay on hot combustion gas (3,000° C.) or hot plasma gas (10,000° C.) can result in overheating of spray particles in flight, which can cause degradation of the sprayed material, and in coatings that are rough, porous and poorly bonded.
These limitations are particularly acute when the polymer being sprayed is a thermoplastic polyamide, such as nylon, or a composition that comprises a polyamide. In fact, because of the potential for thermal degradation of the material, no operable polyamide spray foaming processes have been developed.
The foregoing review of the related art reveals that the problems associated with applying polymer foams and coatings to substrates have not been solved. There is still a need for a thermal spraying process that allows economical application of polymer foams and coatings to substrates that does not result in degradation of the foam or coating material and applied foams and coatings that are of poor quality.
Thermal spray technology is used extensively to apply ceramic and metal coatings. Some thermal spray techniques have been used to form polymer coatings. The applicants are aware of three patents that cover several of the traditional thermal spray techniques: U.S. Pat. No. 4,911,956 for spraying molten thermoplastic, U.S. Pat. No. 5,041,713 for plasma flame spray of polymers and U.S. Pat. No. 5,285,967 for high velocity oxygen fuel thermal spray of plastic coatings. These thermal spray methods and systems in the related art all use plasmas and combustion processes to melt the thermoplastic being sprayed.
Thermal spray techniques have not been used in the background art to apply foams. Current foams of this type require closed, heated and pressurized molds to convert the constituent materials into useful foams.
The background art is characterized by U.S. Pat. Nos. 2,434,911; 2,530,186; 3,677,471; 3,873,024; 3,958,758; 4,065,057; 4,289,807; 4,688,722; 4,835,022; 4,911,956 4,940,623; 5,021,259; and 5,041,713; 5,356,958; 5,503,872; 5,718,864; 6,074,194; 6,342,272; 6,478,234; 6,488,773; and 6,793,976; U.S. Statutory Invention Registration No. H2035; and U.S. Patent Application Nos. 2001/0055652; 2002/0110682; 2003/0207145; and 2003/0209610; the disclosures of which patents, invention statutory registrations and patent applications are incorporated by reference as if fully set forth herein.
In U.S. Pat. No. 2,434,911 Denyssen discloses a heating and spraying device. This device is limited in that it is configured to spray a low melting point metal alloy. The alloy enters the nozzle of the device under the force of gravity.
In U.S. Pat. No. 2,530,186 Trimm et al. disclose a portable apparatus for spraying molten metals. This invention is limited in that it is configured to spray a low melting point metal alloy. The alloy enters the nozzle of the device under the force of gravity.
In U.S. Pat. No. 3,677,471 Deakin discloses and apparatus and process for applying coatings. This invention is limited in that it relies on an unreliable heated transport tube arrangement. With this arrangement, many polymer powders of interest will melt in the tube during transport, preventing the delivery of more material. Moreover, the device does not allow the operator to control the temperature of the polymer during transport separate from the temperature of the polymer during spray.
In U.S. Pat. No. 3,873,024 Probst et al. discloses an apparatus for spraying different powders. This invention is limited in that structures are not provided to melt the powders in flight. Moreover, the jets introduced directed at the powder flow are oriented so as to create a helical powder flow.
In U.S. Pat. No. 3,958,758 Piorkowski discloses a spraying apparatus. This invention is limited in that a divergence nozzle is discloses which requires large amounts of added air to operate. Moreover, the device is configured to create suction at two stages: initially to draw in the polymer powder and later to draw in the heated air to process the powder. All this additional air intake requires greater heating capability.
In U.S. Pat. No. 4,065,057, Durmann discloses an apparatus for spraying heat responsive materials. This invention is limited in that it does not disclose radiant heating means and does disclose a mixing zone that is not large enough to allow complete interaction of the hot air with the sprayed polymer, which can lead to an incomplete melt. Moreover, the invention exposes an excessive amount of the transport tube and spray nozzle to the high temperatures created by electric heating elements. This can lead to the buildup of molten material inside the transport tube and greatly increases the possibility of material buildup on the nozzle.
In U.S. Pat. No. 4,289,807, Christensen et al. disclose an apparatus and method for fusion processing of thermoplastic resins. This invention is limited in that a single stream of superheated steam is used as the transport medium.
In U.S. Pat. No. 4,688,722, Dellassio et al. disclose a nozzle assembly for a plasma spray gun. This invention is limited in that its structures are appropriate for plasma flame spraying.
In U.S. Pat. No. 4,835,022, Huhne discloses a process and apparatus for coating components. This invention is limited in that its structures are appropriate for plasma flame spraying.
In U.S. Pat. No. 4,940,623, Bosna et al. disclose a thermal spray technique. This invention is limited in that it relies on the use of broken hollow glass microspheres. Moreover, the device is configured to spray molten copper particles.
In U.S. Pat. No. 5,021,259, Singelyn discloses a method for applying a continuous thermoplastic coating. This invention is limited in that it relies on the presence of an intermediate layer to improve the bonding of a thermoplastic to a metal substrate. The intermediate layer comprises a semi porous layer of a thermal sprayed material that is known to have good adhesion to the metal substrate. That semi-porous layer is then coated with the thermoplastic and subsequently reheated to achieve a pin-hole free coating.
In U.S. Pat. No. 5,356,958, Mathews discloses an impact resistant thermoplastic syntactic foam composite. This invention is limited in that it relies on using thermoset and thermoplastic binders with hollow glass or hollow carbon micro-spheres to create a syntactic foam core. The core material is produced using compression molding and a thermal spray technique is not disclosed.
In U.S. Pat. No. 5,503,872, MacKenzie et al. disclose a flameless plastic coating apparatus and method. This invention is limited in that it is configured such that heated air produced by combustion surrounds the tube through which powder is supplied to the apparatus and cooling air must be used to cool the tube.
In U.S. Pat. No. 5,718,863, McHugh et al. disclose a spray forming process. This invention is limited in that a liquid material is used as the input and a single stream of heated gas is used as the transport medium.
In U.S. Pat. No. 6,074,194, McHugh discloses a spray forming process. This invention is limited in that a liquid material is used as the input and a single stream of heated gas is used as the transport medium.
In U.S. Pat. No. 6,342,272, Halliwell discloses a multi-layer corrosion resistant coating. This invention is limited in that it requires the embedding of particles in a first coating before a second coating is applied.
In U.S. Pat. No. 6,478,234, Klein et al. disclose an adjustable injector assembly for melted powder coating deposition. This invention is limited in that requires that a rotatable powder injector be incorporated into a plasma gun.
In U.S. Pat. No. 6,488,773, Ehrhardt et al. disclose an apparatus and method for spraying a polymer. This invention is limited in that the polymer must be in a molten state before the spray operation begins.
In U.S. Pat. No. 6,793,976, Klein et al. disclose an apparatus and method for polymer spraying. This invention is limited in that no details for the construction of a spray gun are presented and the resulting protective layer is essentially free of voids.
In U.S. Statutory Invention Registration No. H2035, Halliwell discloses a method for applying a polymer coating to a substrate. This invention is limited in that a non-polar powder and a non-oxidizing shielding gas or a reducing gas must be used.
In U.S. Patent Application No. 2001/0055652, Dalzell, Jr. et al. disclose a method for making an abradable seal. This invention is limited in that no details of a plasma spray gun are disclosed.
In U.S. Patent Application No. 2002/0110682, Brogan discloses a non-skid coating. This invention is limited in that no details are provided on the thermal spray gun.
In U.S. Patent Application No. 2003/0207145, Anderson et al. disclose a method for adhering a sold polymer to a substrate. This invention is limited in that an intermediate adhesion promoting polymer coating of metal oxide is required.
In U.S. Patent Application No. 2003/0209610, Miller et al. disclose a high velocity oxygen fuel method for spray coating non-melting polymers. This invention is limited in that a torch and an oxygen fuel is required.
The above review of the background art reveals that problems remain in the art of thermal spray formation of foams and other coatings. There is still a need for a system and method for forming spray-in-place foams and coatings that are similar in performance to the molded foams and other coatings currently in use.